[ARM] pxa: enable L2 if present in XSC3

Check whether L2 is present or not in XSC3. If it's present, enable L2
immediately.

Disabling L2 after L2 is enabled that would result in unpredicatable behavior
of XSC3 processor.

Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>